UNIT 3: ADVANCED MACHINING PROCESSES & MICROFABRICATION

1.0 INTRODUCTION TO NON-TRADITIONAL/ADVANCED MACHINING PROCESSES

1.1 Definition and Need

Advanced Machining Processes (AMPs) are non-conventional manufacturing techniques that use directed energy (mechanical, thermal, electrochemical, chemical) to remove or add material, enabling the machining of hard, brittle, complex-geometry materials impossible with traditional tooling.

Need:

• Machine hard, brittle, heat-sensitive materials (ceramics, composites, semiconductors).

• Achieve complex shapes, micro-features without tool wear.

• Reduce mechanical stresses, thermal damage.

• Enable rapid prototyping, micro/nano-fabrication.

1.2 Classification by Energy Source/Mechanism

1.3 Comparison with Conventional Machining

1.4 Selection Criteria

• Material properties (hardness, brittleness, conductivity).

• Geometric complexity (internal features, aspect ratio).

• Required accuracy & surface finish.

• Production volume (prototype vs. batch).

• Cost constraints (equipment, consumables, post-processing).

[!TIP]

Exam Focus: Expect questions comparing AMPs with conventional machining. Highlight tool wear independence and hard material machining as key advantages.

2.0 ULTRASONIC MACHINING (USM)

2.1 Working Principle & Mechanism

• Principle: High-frequency (15–40 kHz) low-amplitude (10–50 µm) vibrations of tool in abrasive-slurry medium cause micro-chipping.

• Mechanism:

  1. Tool vibrates axially, impacting abrasive grains (SiC, Al₂O₃) in slurry.

  2. Grains strike workpiece surface, inducing micro-cracks.

  3. Repeated impacts cause brittle fracture (chipping) of workpiece material.

  4. Slurry carries debris away.

2.2 Main Components

1. Transducer (Piezoelectric/Magnetostrictive): Converts electrical signal to mechanical vibration.

2. Tool: Soft metal (copper, steel), shaped as desired cavity.

3. Abrasive Slurry: Water-based suspension of abrasive grains.

4. Fixture: Holds workpiece, applies static load.

5. Amplifier & Generator: Drives transducer at resonant frequency.

2.3 Process Parameters & Influence

2.4 Applications

• Brittle & Hard Materials: Ceramics, glass, quartz, semiconductors (Si, Ge), carbides.

• Complex Cavities: Holes, slots, profiles in hard materials.

• Delicate Parts: Thin-walled, fragile components (no mechanical stress).

• Non-conductive Materials (unlike EDM).

2.5 Advantages & Limitations

[!TIP]

Common Pitfall: USM is NOT for ductile metals (they deform, not chip). Remember: brittle fracture is key.

3.0 ELECTRICAL DISCHARGE MACHINING (EDM)

3.1 Basic Working Principle

Thermal Erosion: Repeated electrical sparks between tool (cathode) and workpiece (anode) in dielectric fluid vaporize/ melt material. Dielectric flushes debris and quenches spark.

3.2 EDM System Components

1. Power Supply: Pulse generator (DC, low voltage, high current).

2. Dielectric Fluid: Kerosene, deionized water (insulates, flushes, cools).

3. Tool Electrode: Graphite, copper, tungsten (shaped as desired cavity).

4. Workpiece: Conductive material.

5. Flushing System: Pressure/flow to remove debris.

3.3 Mechanism of Material Removal

1. Spark Generation: Voltage breakdown across smallest gap (ionized dielectric).

2. Crater Formation: Localized melting/vaporization → crater on workpiece.

3. Dielectric Recovery: Fluid enters gap, extinguishes spark, flushes debris.

4. Cycle Repeats: Thousands of sparks/sec.

3.4 Types of EDM

3.4.1 Die-Sinking EDM (Ram EDM)

• Tool electrode machined into workpiece to create negative impression.

• Used for molds, dies, complex cavities.

3.4.2 Wire EDM (Wire Cut EDM)

• Thin wire (brass, copper, tungsten) acts as continuously moving electrode.

• Working Principle: Wire traverses along programmed path, sparking erodes workpiece. Wire never contacts workpiece; gap ~0.01–0.05 mm.

• Wire Materials:

  • Brass: General purpose, good conductivity, cheap.

  • Copper: Higher MRR, softer, more wear.

  • Tungsten: High wear resistance, for hard materials.

• Applications:

  • Precision cutting of hard metals (tool steels, carbides).

  • Complex 2D/3D shapes (extrusions, gears, prototypes).

  • Narrow slots, fine features (<0.1 mm).

  • No tool change for multiple parts.

3.5 Process Parameters

3.6 Performance Characteristics

• MRR: Volume removed per unit time (mm³/min). Increases with current, pulse on-time.

• TWR: Tool wear volume per unit time. Tool wear ratio (TWR/MRR) critical.

• Surface Roughness (Ra): Increases with current, pulse duration.

• Accuracy: ±0.005–0.05 mm; affected by tool wear, thermal distortion, taper.

3.7 Errors & Accuracy Issues

3.8 Applications

• Tool & Die Industry: Molds, dies, fixtures.

• Aerospace: Turbine blades, hard alloy components.

• Medical: Surgical tools, implants (Ti, Co-Cr).

• Prototyping: Complex shapes from hard materials.

• Small Holes: Drilling in hard metals (e.g., turbine blades).

3.9 Advantages & Limitations

[!TIP]

Key Formula: Material Removal Rate (MRR) ∝ Current × Pulse On-time. Higher current → higher MRR but worse surface.

4.0 LASER BEAM MACHINING (LBM)

4.1 Principle of Operation

Concentrated thermal energy from laser beam melts/vaporizes material. Focused beam (spot size ~0.01–0.1 mm) delivers high power density (10⁶–10⁹ W/cm²).

4.2 Types of Lasers Used

4.3 Laser Beam Generation & Delivery

• Pumping: Optical/electrical energy excites lasing medium.

• Resonator: Mirrors amplify beam.

• Delivery: Mirrors/fibers guide beam to focusing lens.

• Focusing: Lens concentrates beam to minimum spot size at focal point.

4.4 Mechanism of Material Removal

• Melting & Vaporization: For metals, polymers.

• Thermal Stress: For brittle materials (glass, ceramics) → crack propagation.

• Photochemical Ablation: For excimer lasers (UV breaks molecular bonds).

4.5 Process Parameters

4.6 Effect of 'Focusing' on Performance

• Spot Size: Minimum at focal point. Smaller spot → higher power density.

• Power Density: $$\displaystyle P_d = \frac{P}{\pi r^2} $$ (P = power, r = spot radius). ↑ $$\displaystyle P_d $$ → efficient material removal.

• Kerf Width: ≈ spot size; defocusing increases kerf.

• HAZ (Heat Affected Zone): Minimal at tight focus; defocusing increases HAZ.

• Depth of Focus: Range where spot size remains small; affects cut depth consistency.

4.7 Applications

• Cutting: Sheet metal, plastics, ceramics.

• Drilling: Micro-holes (0.01–1 mm) in turbine blades, fuel injectors.

• Welding: Precision, deep penetration (keyhole welding).

• Marking/Engraving: Barcodes, serial numbers.

• Micromachining: MEMS, semiconductor processing (excimer).

4.8 Advantages & Limitations

[!TIP]

Remember: Excimer lasers (UV) are for cold ablation (minimal HAZ), ideal for biomaterials & polymers.

5.0 RAPID PROTOTYPING (RP) / ADDITIVE MANUFACTURING (AM)

5.1 Definition & Significance

RP/AM: Layer-by-layer fabrication of physical models from 3D CAD data. Significance:

• Reduced Lead Time: From weeks to days/hours.

• Design Iteration: Fast validation, testing.

• Complex Geometries: Lattices, internal channels impossible with subtractive.

• Customization: Low-cost one-off parts, medical implants.

5.2 Classification (ASTM F2792)

5.3 Starting Materials

1. Polymers: Thermoplastics (ABS, PLA, Nylon), Photopolymers (UV-curable resins).

2. Metals: Powders (stainless steel, Ti-6Al-4V, Al), Wires (for DED).

3. Ceramics & Composites: Powder-based (SLS), slurry-based.

4. Paper/Composites: For LOM (paper + adhesive).

5.4 Working Principles of Key Processes

5.4.1 Fused Deposition Modeling (FDM)

• Thermoplastic filament melted in nozzle, extruded layer-by-layer.

• Supports (same/different material) for overhangs.

• Post-processing: Remove supports, sanding.

5.4.2 Stereolithography (SLA)

• UV laser cures liquid photopolymer resin vat layer-by-layer.

• Platform lifts, resin flows, recoater smooths surface.

• Highest accuracy & surface finish among polymers.

• Post-curing required.

5.4.3 Selective Laser Sintering (SLS)

• Laser sinters (fuses) powder particles (nylon, polyamide) in bed.

• No supports needed (unsintered powder acts support).

• Strong, functional parts; rough surface.

5.4.4 3D Printing (Inkjet-based)

• Material Jetting: Printheads deposit photopolymer droplets, UV-cured.

• Multi-material/color possible (PolyJet).

• High resolution, smooth finish.

5.5 Process Parameters & Influence

5.6 Application Issues in RP

5.7 Industrial Applications

• Prototyping: Design verification, fit/function testing.

• Tooling: Molds, jigs, fixtures (direct metal printing for conformal cooling).

• Customized Parts: Medical implants (patient-specific), aerospace brackets.

• Biomedical: Surgical guides, tissue scaffolds, drug delivery devices.

• Architecture: Scale models, complex facades.

5.8 Advantages & Limitations

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Stratified Wire: Interpreted as layered wire-based AM (e.g., Wire Arc Additive Manufacturing - WAAM) or sheet lamination (LOM). In RP context, it refers to layer-by-layer deposition of wire/material to build 3D shape.

6.0 MICROFABRICATION & MICROSYSTEMS

6.1 Definition & Need

Microfabrication: Fabrication of micro-scale (1–1000 µm) structures/devices. Need: Miniaturization for electronics, biomedical, optics → higher density, lower cost, new functionalities (MEMS/NEMS).

6.2 Basic Types of Microsystem Devices (MEMS/NEMS)

6.3 Microfabrication Processes

6.3.1 Bulk Micromachining

• Definition: Etching into single-crystal substrate (Si, quartz).

• Process: Masking → Wet/Dry etching → Structures released.

• Example: Pressure sensors (Si diaphragm).

6.3.2 Surface Micromachining

• Definition: Build structures on top of substrate by depositing/etching thin films.

• Process: Deposition (CVD, PVD) → Patterning (photolithography) → Etching → Sacrificial layer removal.

• Example: MEMS mirrors, accelerometers.

6.3.3 LIGA Process

• Acronym: Lithography, Electroplating, Molding.

• Steps:

  1. Deep X-ray Lithography: Use synchrotron X-rays to expose thick PMMA resist → high aspect ratio mold.

  2. Electroplating: Ni, Au plated into mold → strong, conductive microstructures.

  3. Molding: Replicate plastic/metal parts via injection molding or hot embossing.

• Advantages: High aspect ratio (>100:1), vertical sidewalls, metallic parts.

• Applications: Micro-optics, connectors, fuel injection nozzles.

6.4 Industrial Applications

• Electronics: Inkjet printheads, RF switches, memory.

• Medical: Drug delivery (microneedles), diagnostics (lab-on-chip), stents.

• Automotive: Airbag accelerometers, tire pressure sensors.

• Optics: Micro-lenses, optical switches, displays.

• Aerospace: Inertial navigation systems, flow sensors.

6.5 Challenges in Microfabrication

• Material Selection: Limited to Si, polymers, metals; biocompatibility needed.

• Aspect Ratio: High AR structures prone to stiction, collapse.

• Surface Effects: Stiction (capillary forces), adhesion, friction dominate at micro-scale.

• Packaging: Hermetic sealing, biocompatibility, integration with macro-world.

• Cost: Masks, cleanroom, specialized equipment (LIGA needs synchrotron).

[!TIP]

LIGA Key Point: Enables high-aspect-ratio metallic microstructures via X-ray lithography (not UV). Used for mass production via molding.

7.0 ELECTROCHEMICAL MACHINING (ECM) & RELATED PROCESSES

7.1 ECM Principle

Anodic Dissolution: Workpiece (anode) material is atomically removed by controlled electrochemical reaction in electrolyte. No tool wear (tool is cathode).

Overall Reaction:

$$ \text{Workpiece (M)} \rightarrow \text{M}^{n+} + n\text{e}^- $$

7.2 ECM System Components

1. Power Supply: DC (low voltage, high current, 0.5–40 V, 100–10,000 A).

2. Electrolyte: Aqueous solution (NaCl, NaNO₃) for conductivity, flushing, heat removal.

3. Tool Cathode: Shaped as desired cavity; made of copper, brass, stainless steel.

4. Workpiece Anode: Conductive material (stainless steel, Ti, Ni alloys).

5. Feed Mechanism: Maintains constant gap (0.1–0.5 mm).

7.3 Mechanism of Material Removal

• Faraday's Law:

$$ \text{MRR} = \frac{I \cdot M}{n \cdot F \cdot \rho} $$

Where:

$I$ = Current (A),

$M$ = Atomic mass (g/mol),

$n$ = Valency,

$F$ = Faraday constant (96,500 C/mol),

$\rho$ = Density (g/cm³).

• Electrochemical Reactions:

  Anode (oxidation): $$\displaystyle \text{M} \rightarrow \text{M}^{n+} + n\text{e}^- $$

  Cathode (reduction): $$\displaystyle 2\text{H}_2\text{O} + 2\text{e}^- \rightarrow \text{H}_2 + 2\text{OH}^- $$

  (in neutral/alkaline electrolytes)

• Debris Removal: Electrolyte flow carries away dissolved ions and hydrogen gas.

7.4 Process Parameters

7.5 Etch Factor (Definition & Significance)

• Definition:

$$ \text{Etch Factor} = \frac{\text{Undercut (lateral erosion)}}{\text{Depth of Penetration}} $$

• Significance:

  • Measures accuracy of ECM (ability to replicate tool shape).

  • Lower etch factor (≈0–0.1) → less undercut, better dimensional accuracy.

  • Higher etch factor → more undercut, poor shape replication.

  • Controlled by electrolyte type (passivating reduces side erosion), gap, current density.

7.6 Electrochemical Honing (ECH)

• Principle: Combines ECM (anodic dissolution) with mechanical honing (abrasive stones).

• Tool: Rotating cathode with abrasive stones.

• Mechanism: ECM removes bulk material; honing stones mechanically finish surface, improve geometry.

• Differences from ECM:

  | Aspect | ECM | ECH | |------------------|----------------------------------|----------------------------------| | Material Removal | Pure electrochemical | ECM + mechanical abrasion | | Surface Finish | Moderate (depends on flow) | Superior (honing action) | | Tool | Simple cathode | Rotating cathode with abrasives | | Application | Roughing, complex contours | Finishing of holes, bores |

• Applications: Precision finishing of cylinder bores, gear teeth, hardened surfaces.

7.7 Applications

• Complex Contours: Turbine blades, internal profiles (no tool wear).

• Hard Materials: Carbides, heat-treated steels, titanium.

• Burr-free Parts: No mechanical stress → no burrs.

• Mass Production: High MRR for hard materials (e.g., die-casting dies).

• Sensitive Parts: Thin-walled, delicate components.

7.8 Advantages & Limitations

[!TIP]

Etch Factor Formula: Always remember: Undercut / Depth. Lower is better for accuracy. Passivating electrolytes (NaNO₃) reduce side erosion.

8.0 PERFORMANCE CHARACTERISTICS & MECHANISMS (Cross-Cutting)

8.1 Material Removal Rate (MRR) Mechanism

General Trend: MRR ∝ Energy Input (current × voltage × time for EDM/ECM; power × time for LBM; amplitude × frequency × load for USM).

8.2 Surface Integrity Aspects

8.3 Tool/Electrode Wear Considerations

• EDM (Die-sinking): Tool wear inevitable; graphite wears less than copper. Corner wear critical.

• Wire EDM: Wire consumable; brass standard, tungsten for hard materials. Breakage risk.

• USM: Tool no wear (soft metal), but deformation possible.

• ECM: Tool no wear (cathode), but passivation or gas coverage can reduce efficiency.

• LBM: No tool (optics only), but lens contamination possible.

8.4 Economics of Advanced Processes

[!TIP]

Exam Strategy: For "compare economics" questions, focus on equipment vs. per-part cost and volume sensitivity. EDM/ECM economical for hard material batch production; RP economical for low-volume complex prototypes.

Final Note: This unit emphasizes energy-based material removal/addition. Key differentiators:

• USM → Brittle fracture, non-conductors.

• EDM → Thermal sparks, conductors only.

• LBM → Thermal concentration, any material (wavelength-dependent).

• ECM → Electrochemical dissolution, no heat, conductors.

• RP → Additive, layer-by-layer, design freedom.

• Microfabrication → Silicon-based, batch processing, high precision.